Many modern electrically scanned antenna arrays for radar, communication and electronic countermeasures systems require large instantaneous bandwidth. Historically, electronically scanned antennas have utilized phase shifters in each element of the array to control beam position and direction. However, in moderate to large arrays, such a method results in a beam position which varies with frequency. This prevents instantaneous operation over a large portion of the bandwidth since the beam position will move away from the desired direction, or the beam pattern becomes distorted.
An alternative to phase shifters which can be used to scan the beam in a frequency independent manner is the use of true time delay circuits, whereby the time delay of a signal is varied rather than the phase. While this approach has been recognized, few practical implementations of this method have been developed. One such method involves the use of fiber optic delay lines whereby a microwave signal is carried on a lightwave whose time delay is varied. After the appropriate delay, the lightwave is detected and converted back to a microwave signal.
However, fiber optic delay lines for wideband microwave array antennas have several disadvantages. First, the microwave signal is modulated onto a lightwave at the input to the fiber (delay line) and then converted back (demodulated) to a microwave signal at the fiber output. These processes result in signal loss which can be as high as 20 to 30 dB. This signal loss must be made up by external amplifiers, which add complexity to the system. In addition, the optical detection process adds noise to the microwave signal which cannot be totally removed.
Some of the optical approaches utilize lasers, whose frequency is varied to provide the variable delay. This approach has limitations in the switching time. Whereas the desired switching time for large array communications is fast for example, one microsecond (e.g. 1 .mu.sec); the achievable time in prior art laser switching devices is relatively slow on the order of 100 msec.
Furthermore, optical approaches tend to be expensive. Since many such devices are required in a typical array (100 to 1000), the cost for producing multiple fiber optic delay lines may be prohibitive.
The approach described here overcomes the shortcomings described above, because all of the time delay is accomplished with microwave circuitry alone, eliminating the need for optical fibers. By eliminating the need to convert from microwaves to light and back again, the large signal loss is eliminated. The approach described here uses microwave switches which are very fast, resulting in switching times of much less than 1 .mu.sec. Finally, by the use of monolithic microwave integrated circuit (MMIC) technology and printed circuit transmission lines, the approach described here can be implemented at low cost.